The present invention relates to a process for producing aluminum support for printing-plate, and particularly relates to a process for producing a printing-plate support constituted by a surface-roughed aluminum plate suitable for an offset printing plate.
It has been proposed to use an aluminum plate (including an aluminum-alloy plate) as a printing-plate support and particularly as a support for an offset printing plate.
Generally, in order to use an aluminum plate as a support for offset printing, it is necessary that the aluminum plate have a suitable adhesion to a photosensitive material and suitable water-retention.
To this end, it is necessary to surface-rough the aluminum plate to achieve a uniformly and finely grained surface. This surface-roughing treatment largely affects the performance and durability of a plate material when offset printing is performed using such a plate. Therefore, the quality of the surface-roughing treatment has been an important factor in producing a plate material.
An AC electrolytic etching method has been used generally for surface-roughing an aluminum printing-plate support. This etching method uses a sinusoidal-wave AC current or a special alternating waveform current such as a square wave. By use of a proper electrode (such as graphite) as an opposite electrode, the surface-roughing treatment is performed on an aluminum plate with an AC current. The treatment is usually performed once, and, generally, the depth of the thus obtained pits has been so shallow that the plate is poor in durability against printing. Various methods have been therefore proposed to obtain an aluminum plate suitable as a printing-plate support having a grained surface where pits sufficiently deep relative to their diameter are uniformly and finely formed. The proposed methods include the following: a surface-roughing method using a special electrolytic power source waveform (see Japanese Patent Unexamined Publication No. 53-67507); a ratio between quantities of electricity in an anode time and in a cathode time in electrolytic surface-roughing by using an alternating current (see Japanese Patent Unexamined Publication No. 54-65607); a power source waveform (see Japanese Patent Unexamined Publication No. 55-25381); a combination of quantities of current conduction per unit area (see Japanese Patent Unexamined Publication No. 56-29699), and so on.
In performing electrochemical surface-roughing on an aluminum-alloy plate represented by a JIS 3003 material, the pits obtained by the above-mentioned methods lack sufficient depth and uniformity and have a complicated uneven shape. Moreover, even if an offset printing plate is formed using such an aluminum-alloy plate, the printing plate is insufficient in printing performance and durability and it has been difficult to obtain a satisfactory printing plate. To solve this problem, an electrochemical surface-roughing using an alternating current of a low frequency within a range of 1.5 to 10 Hz has been disclosed in U.S. Pat. No. 4,482,434. However, where an aluminum plate is subjected to continuous electrochemical surface-roughing by using such a low frequency alternating current, the thus formed printing plate has a defect that in treatment, unevenness in the form of a transverse stripe pattern occurs perpendicularly to the advancing direction of the aluminum plate. Moreover, when a low frequency alternating current is used, carbon conventionally used for electrochemical surface-roughing quickly dissolves and it has been difficult to use this surface-roughing method in industrial practice.